(1) Field of the Invention
The present invention relates to a process for producing a thin sheet of a Cr-Ni-based stainless steel having excellent surface quality and workability, comprising bringing the thickness of a cast material to a thickness close to the thickness of a product through the so called "synchronous continuous casting process" which does not give rise to a difference in the relative speed between a cast material and the inner wall surface of a mold, wherein the structure of the cast material is refined at a casting stage and, at the same time, MnS is precipitated.
(2) Description of the Prior Art
A thin sheet of a stainless steel has hitherto been produced by a continuous casting process that comprises casting a stainless steel into a cast slab having a thickness of 100 mm or more while vibrating a mold in the direction of casting, surface-grinding the slab, heating the grinding slab to 1000.degree. C. or above in a heating furnace, hot rolling the heated slab into a hot strip having a thickness of several millimeters by means of a hot strip mill comprising a rough rolling mill and a series of finishing rolling mills.
In the cold rolling of the hot strip thus obtained, in order to ensure the configuration (flatness), the quality of the material and the surface quality, the hot rolled sheet was annealed for the purpose of softening the hot strip subjected to strong hot working, and scale etc. on the surface of the hot strip was removed by grinding after the step of pickling. This conventional process needed long and large hot rolling facilities and a great amount of energy for heating and working the material. For this reason, this process was also not regarded as an excellent production process from the viewpoint of productivity. Further, since the final product is obtained by subjecting a cast slab having a thickness of 100 mm or more to many working treatments, a texture develops. For this reason, a user should consider an anisotropy in the final product when said final product is press working by the user. Thus, the above process also had many usage limitations
In recent years, studies on a process for producing a cast material (hereinafter referred to as "cast strip") having a thickness equal to or close to the thickness of a hot strip in the course of the continuous casting have developed with a view to solving the problem concerning the necessity of long and large hot rolling facilities and a great amount of energy and rolling power for rolling a cast slab having a thickness of 100 mm or more into a hot strip. For example, an article featured in "Tetsu to Hagane (Iron and Steel)", '85, A197-'85, A256 discloses a process wherein a hot strip is directly produced by continuous casting. In such a continuous casting process (hereinafter referred to as "novel process"), the use of a twin drum system is studied when the gauge of the cast strip to be produced is on the level of 1 to 10 mm, while the use of a twin belt system is studied when the gauge of the cast strip is on the level of 20 to 50 mm.
In these novel processes, however, a problem exists in the stage of casting as well, and problems with respect to the quality of the product and the surface quality remains unsolved.
"CAMP ISIJ", vol. 3, 1990, p. 770 reports that a surface defect in an orange peel form on the surface of the cast strip occurs, therefore the .gamma. grain diameter in the cast strip before could rolling must be refined
An article featured in "CAMP ISIJ", vol. 1, 1988, p. 1674-1705 describes that a SUS304 thin sheet product obtained by a twin drum casting and a single cold rolling process has a finer grain structure than that produced by the conventional process and has a low elongation and, in order to prevent this phenomenon, the cast strip is annealed at a high temperature for a long period of time to remove the .delta. ferrite remaining in the cast strip.
In a process of which the development has been advanced as a novel process on the conditions that a cast strip having a thickness equal to or close to a hot strip is produced by continuous casting, since the steps from casting to the final product are simplified, the surface properties and workability of a thin sheet of stainless steel are greatly influenced by the properties of the cast strip. That is, in order to prepare a product having excellent surface quality and workability, it is necessary to prepare an excellent cast strip.
The present inventors have clarified the means for refining a .gamma. grain of a cast strip in a method for preventing the occurrence of a roping phenomenon or the above described surface defect called "orange peel phenomenon" in the novel process and in previously filed patent applications.
Japanese Patent Application No. 63-221471: PA0 Japanese Patent Application No. 63-169095 (PCT International Publication No. WO 90/00454): PA0 Japanese Patent Application No. 63-221472: PA0 Japanese Patent Application No. 63-286690: PA0 Japanese Patent Application No. 1-1586:
Cooling of cast strip and hot rolling PA1 Cooling of a cast strip and cold rolling and annealing in a two stage (2CR) process PA1 Cooling of a cast strip and .delta. ferrite control PA1 Cooling of a cast strip and .delta. ferrite control PA1 Cooling of a cast strip and the addition of a grain refining element
It has become apparent that in these processes alone, the elongation and workability, which are features of the Cr-Ni-based stainless steel, become unsatisfactory.
In order to solve the above described problem, the present inventors have made detailed studies on a process for producing a thin sheet of a Cr-Ni-based stainless steel. As a result, they have found that the .delta. ferrite remaining and fine MnS in a cast strip suppress the growth of recrystallized grain in the annealing after cold rolling and this is causative of a fine grain structure of the final product and a lowering in the elongation attributable thereto, and propose holding the material in a temperature range of from 800.degree. to 1250.degree. C. for the purpose of reducing the .delta. ferrite and the same time precipitating MnS coarsely (see Japanese Patent Application No. 2-83024 (PCT International Publication No. WO 91/10517)). Since, however, in the above-described technique, the cast strip should be held in the above described temperature range for 80 min or less (in the working examples, the maximum holding time and the minimum holding time are 900.degree. C..times.60 min and 1200.degree. C..times.3 min, respectively), it becomes necessary to use long and large holding facilities, which renders the above-described technique unsuitable for practical use.
Further, the present inventors have studied the cooling behavior of the cast strip produced by the above described process and the coarsening behavior of MnS through precipitation and have clarified the requirement for coarsening of MnS, which has led to a solution to a above described problem.